A bandgap reference voltage circuit provides an output reference that is insensitive to temperature, supply voltage and process variations. Bandgap reference voltage circuits are used in a wide variety of electronic circuits, such as wireless communication devices, memory devices, voltage regulators, filters, analog-to-digital converters, digital-to-analog converters and so on.
There are a variety of ways to generate a bandgap reference voltage that, in general, fall into two main categories. The first category is current mode generation of a bandgap reference voltage. The second category is voltage mode generation of a bandgap reference voltage.
In current mode generation of a bandgap reference voltage, current from a first current source that has a positive temperature coefficient is summed with current from a second current source that has a corresponding negative temperature coefficient to produce an output current. A bandgap reference voltage (VREF) is achieved by passing the output current through a resistance. The use of corresponding positive and negative temperature coefficients in the first and second current sources results in the effects of temperature being canceled out, thus stabilizing the bandgap reference voltage (VREF). For examples of bandgap reference voltage circuits that use current mode generation, see U.S. Pat. No. 5,666,046 and United States Patent Application number 2004/0155700A1.
In voltage mode generation of a bandgap reference voltage, a base-emitter voltage (VBE) is generated from a bipolar transistor having a negative coefficient. A thermal voltage (Vt) is generated that is proportional-to-absolute-temperature (PTAT). The thermal voltage (Vt) has a positive coefficient at room temperature. The thermal voltage is equal to the expression kT/q where k is Boltzmann's constant, T is absolute temperature, and q is the elementary electron charge constant. Neither k nor q is temperature-dependent, and the result is that thermal voltage (Vt) is directly proportional-to-absolute-temperature (PTAT). The thermal voltage (Vt) is multiplied by a constant (K) and summed with the voltage (VBE). The result is a reference voltage (VREF) that is described by Equation 1 below:VREF=VBE+KVt  EQUATION 1
For examples of voltage mode generation of a bandgap reference voltage, see for example, U.S. Pat. No. 4,849,684 and U.S. Pat. No. 5,900,773.
Current mode generation of a bandgap reference voltage has the ability to achieve a bandgap reference as low as approximately 1 volt. Additionally, current mode generation of a bandgap reference voltage can produce a programmable bandgap reference voltage as the output current has zero temperature coefficient. However, current mode generation of a bandgap reference voltage typically requires one or more relatively large resistors, which can result in a large chip size.
Voltage mode generation of a bandgap reference voltage requires smaller total resistance than current mode generation of a bandgap reference voltage, but typically cannot be used for generating a bandgap reference voltage less than about 1.2 volts. Also, using the voltage mode generation of a bandgap reference voltage, the resulting bandgap reference voltage typically is 1.2 volts or a positive integer multiple of 1.2 volts.